It is well known that electrically conductive, fluorine-doped tin oxide layers on glass surfaces decrease the electrical resistance of the surfaces so coated, as well as increase the infrared reflection. To produce such layers, suitable tin compounds (basic compounds) along with a fluorine-emitting compound (doping agent) are brought simultaneously into contact with the glass surface that has been heated to 400.degree. to 800.degree. C. The basic tin compound forms a coherent tin oxide layer on the surface of the glass, the glass-ceramic or the enamelization. The fluorine from the doping agent increases the electric conductivity and brings about the high infrared reflection.
From an engineering point of view, the spraying of suitable tin-containing and fluorine-containing solutions for applying the fluorine-doped tin oxide layers on the surfaces is particularly simple.
A liquid preparation for producing high-grade, fluorine-doped tin oxide layers on glass surfaces is described in the European publication 0 158 399. The preparation consists of
a) 1 to 30% by weight of a doping agent based on an organic fluorine compound, selected from trifluoroacetic acid or its anhydride, ethyl trifluoroacetate, trifluoroethanol and pentafluoropropionic acid, and PA1 b) 70 to 99% by weight of an organic tin compound, selected from alkyl tin trichloride, dialkyl tin dichloride, alkyl dichloro tin acetate, alkyl chloro tin diacetate, an ester of tin chloride or tin tetrachloride. PA1 (i) the preparation shall be homogeneous and already contain the doping agent; PA1 (ii) the preparation shall be stable for a long time and be readily dilutable with alcohols, ketones, esters and other polar solvents; PA1 (iii) the chloride content of the preparation shall be minimized; and PA1 (iv) The tin oxide layers, produced on the substrates with the preparations, shall have a high transparency in the visible light wavelength region, as low a surface resistance as possible and a high reflection in the infrared wavelength region.
This method, however, is not satisfactory, since the fluorine-containing doping agents, because of their low boiling point, largely evaporate ineffectively at the hot glass surface and thus are not incorporated in the tin oxide layer. In addition, the hot surface of the substrate is strongly cooled by this effect. This is disadvantageous for a homogeneous layer build-up and further deteriorates the optical and functional properties.
The European patent 0 312 879 discloses a preparation, which consists of tin tetrachloride or alkyl tin trichloride with tin(II) fluoroborate in a polar organic solvent.
As doping agent, tin(II) fluorosilicate is used according to the German patent 37 35 591 and Tin(II) fluoride is used according to the German patent 37 35 574.
However, the high chlorine content is the main disadvantage of these preparations. One of the products of pyrolysis is hydrogen chloride, which represents a danger to health and can attack and damage materials.
The invention is concerned with the technical problem of finding a preparation which has the following properties profile: